Sometimes I do, sometimes I don't. It all depends on the fabrics and how I'm going to use them. I have a dryer with a wire rack that I can use to dry things so they don't tumble. I've even prewashed (or more like presoaked) fat quarters ad put them on the rack to dry.
I come from a garment construction background and this is all new ground to me, too. I do a lot wearable art and those fabrics don't need to be prewashed as they will never be washed. Other things are a bit case by case.